Although the piston damper is the same as a rotary damper, for example, in terms of applying a brake on the movement speed of the second member relative to the first member, the piston damper can easily be assembled without the need for a gear and the like compared to the rotary damper. As one example thereof, FIGS. 11(a) and 11(b) show a structure of a piston damper (an air damper) disclosed in Patent Document 1. FIG. 11(a) shows a usage example of the piston damper (air damper), and FIG. 11(b) shows the structure of the piston damper. The piston damper D comprises a cylinder 11, a piston 31 reciprocally disposed inside the cylinder 11, and a rod 41 moved synchronously with the piston 31. Also, on an external surface of the cylinder 11, there is provided an attachment portion (an attachment piece) 18 wherein a fitting hole (an attachment hole) 19 is formed. Also, in an end portion of the rod 41, there is provided a clamp-type attachment portion (a locking portion) 42 wherein one portion of a hole is open. Then, in the piston damper D, the cylinder 11 is axially supported on a disposed portion on an instrument panel side by an axis of a screw, a bolt, and the like, which are not shown in the figures, using the fitting hole 19. The rod 41 is axially supported on a disposed portion on a glove box G side by an axis of an axis-like projection, the bolt, and the like, which are not shown in the figures, using the attachment portion 42. Thereby, when a glove box G is switched from a state locked by a lock device at a closed position to an open position under its own weight by being unlocked by a press operation of an operation button B, the glove box G slowly turns while being dampened by the piston damper D.
In the aforementioned piston damper D, however, there is formed a housing groove 32 around the piston 31, and an O-ring 51 is housed in the housing groove 32. Also, there is provided an orifice (a first orifice) 35 using the housing groove 32. There is a possibility that the O-ring 51 can be twisted when the piston 31 moves in an axial direction inside the cylinder 11. When the O-ring 51 is twisted in such a manner, an external diameter of the O-ring 51 becomes smaller than an external diameter before the O-ring 51 is twisted so as to impair a braking action of the piston damper D. Therefore, in the piston damper D shown in FIG. 11(a) and FIG. 11(b), there is provided a fixed portion (a first encircling wall 33, and a projecting wall portion which is not shown in the figures) for a movement control corresponding to the O-ring 51 in a portion separated from the first orifice 35 in the piston 31 so as to prevent the O-ring 51 from twisting. Thereby, the braking action of the piston damper D can be obtained over a long period of time.